A circuit arrangement for switching-over a bistable relay with the aid of a semiconductor is, for example, known from the book "Relais Lexikon" (Relay Lexicon) by H. Sauer, first edition, 1975, page 12. Upon application of an excitation voltage, a first transistor connected in series with the relay coil and a capacitor is conductive, the relay is actuated and the capacitor is charged. When a positive control signal is provided at the input of a second transistor, the first transistor is blocked, and a third transistor connected in parallel to the coil and the capacitor is rendered conductive. The capacitor is discharged through this third transistor and the relay switches back. If the control signal jumps to zero value, then the second and third transistors are again blocked, the first transistor becomes conductive and the capacitor is again charged, so that the relay switches over.
A circuit arrangement of this type is expedient for this operation of bistable relays when the polarity of the excitation voltage remains unchanged. The relay remains in its switched position after the capacitor has been charged, independent of whether the excitation voltage is switched off or continues. A diode prevents slow discharging of the capacitor when the excitation voltage is absent. The relay may be switched back only by a positive control pulse at the input of the second transistor. Since this pulse cannot be produced from the excitation voltage when the latter is switched off, an external control signal source is necessary.
Furthermore, a circuit arrangement for controlling a bistable relay is known from German published patent application No. 2 624 913, in which this circuit arrangement acts as a monostable relay and thus switches back automatically to the starting position upon insufficiency of the excitation voltage. This is attained by connecting an evaluation circuit fed with the excitation voltage between the junction of the coil and the capacitor, on the one hand, and the control electrode of the semiconductor switch, on the other hand, the evaluation circuit blocking the semiconductor when the excitation voltage is present and rendering the semiconductor conductive when such voltage is absent. For preventing inadvertent discharge of the capacitor, a diode is also connected in the current path. A series resistance in the same path serves for short-circuit-proofing the semiconductor as well as for realizing--if properly sized--a defined voltage drop, so that the relay may be provided with economic low-voltage winding and may be operated at higher voltages, such as line voltage. However, the evaluation circuit required to achieve the monostable function of the relay leads to a relatively high expenditure of components.